Bacterial infections caused by staphylococcus bacteria (i.e., a “staph infection”) are very common in the general population. About 25% of individuals commonly carry staphylococcus bacteria on their skin or in their nose. Most of the time, these bacteria cause no problem or result in relatively minor skin infections. However, staph infections can turn deadly if the bacteria invade deeper into an individual's body, for example, entering the bloodstream, joints, bones, lungs or heart. In the past, a lethal staph infection might have occurred in a person who was hospitalized or had a chronic illness or weakened immune system. Now, it is increasingly common for an otherwise healthy individual to develop life-threatening staph infections. Importantly, many staph infections no longer respond to common antibiotics.
Staphylococcus aureus, often referred to as “staph,” Staph. aureus,” or “S. aureus,” is a major human pathogen, producing a multitude of virulence factors making it able to cause several types of infection, from superficial lesions to toxinoses and life-threatening systemic conditions such as endocarditis, osteomyelitis, pneumonia, meningitis and sepsis (reviewed in Miller and Cho, “Immunity Against Staphylococcus aureus Cutaneous Infections,” Nat. Rev. Immunol. 11:505-518 (2011)). Although most individuals encounter S. aureus shortly after birth (Holtfreter et al., “Towards the Immune Proteome of Staphylococcus aureus—The Anti-S. aureus Antibody Response,” Int. J. Med. Microbiol. 300:176-192 (2010)) and possess both antibodies against S. aureus and the ability to increase anti-S. aureus titers after infection, these antibodies are often not protective against recurrent S. aureus infections (Foster T J, “Immune Evasion by Staphylococci,” Nat. Rev. Microbiol. 3:948-958 (2005)). In the United States alone, an annual mortality of more than 20,000 is attributed to methicillin-resistant S. aureus (MRSA), exceeding deaths caused by influenza, viral hepatitis, and HIV/AIDS (Foster, T J., “Immune Evasion by Staphylococci,” Nat. Rev. Microbiol. 3:948-958 (2005); Klevens et al., “The Impact of Antimicrobial-Resistant, Health Care-Associated Infections on Mortality in the United States,” Clin. Infect. Dis. 47:927-930 (2008)). The pathogen produces a variety of molecules that presumably facilitate survival in or on the human host.
Bi-component, pore-forming leukotoxins are among the secreted virulence factors produced by S. aureus. These toxins can be secreted as water soluble monomers which oligomerize, and then insert pores into the plasma membrane, which subsequently disrupt the cellular osmotic balance and membrane potential leading to death of the targeted cells, most notably polymorphonuclear leukocytes (PMNs) and mononuclear phagocytes (Bischogberger et al., “Pathogenic Pore-Forming Proteins: Function and Host Response,” Cell Host Microbe 12(3):266-275 (2012), which is hereby incorporated by reference in its entirety). In the case of Leukotoxin ED (LukED), the targeting, binding, and killing of host phagocytic cells occurs via the cellular target CCR5, CXCR1 and CXCR2 located on the surface of the phagocytes (Alonzo III et al., “Staphylococcus aureus Leucocidin ED Contributes to Systemic Infection by Targeting Neutrophils and Promoting Bacterial Growth In Vivo,” Mol. Microbiol. 83:423-435 (2012); Alonzo III et al. “CCR5 is a Receptor for Staphylococcus aureus Leukotoxin ED,” Nature 493(7430)51-55 (2012); and Reyes-Robles et al., “Staphylococcus aureus Leukotoxin ED Targets the Chemokine Receptors CXCR1 and CXCR2 to Kill Leukocytes and Promote Infection,” Cell Host & Microbe 14:453-459 (2013)). Indeed, when the cellular target of LukED, CCR5, is not present on host immune cells, the host animal is resistant to the otherwise lethal S. aureus infection (Alonzo III et al. “CCR5 is a Receptor for Staphylococcus aureus Leukotoxin ED,” Nature 493(7430):51-55 (2012)). Leukotoxin AB (LukAB) can also kill host phagocytic cells, and its cytolytic activity can be exerted both from the outside and from the inside of the cell, after the microorganism is phagocytosed into the host cell (Dumont et al., “Staphylococcus aureus LukAB Cytotoxin Kills Human Neutrophils by Targeting the CD11b Subunit of the Integrin Mac-1,” PNAS 110(26):10794-10799 (2013)). Due to the contribution that both of these leukotoxins have to pathogenesis, they have been considered critical S. aureus virulence factors (Alonzo III and Tones, “Bacterial Survival Amidst an Immune Onslaught: The Contribution of the Staphylococcus aureus Leukotoxins,” PLOS Path 9(2):e1003143 (2013)).
Another critical factor for the pathogenic success of S. aureus depends on the properties of its surface proteins (Clarke et al., “Surface Adhesins of Staphylococcus aureus,” Adv. Microb. Physiol. 51:187-224 (2006); Patti et al., “MSCRAMM-Mediated Adherence of Microorganisms to Host Tissues,”Annu. Rev. Microbiol. 48:585-617 (1994); and Patti et al., “Microbial Adhesins Recognizing Extracellular Matrix Macromolecules,” Curr. Opin. Cell Biol. 6:752-758 (1994)).
S. aureus employs microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) in order to adhere to and colonize host tissues. MSCRAMMs can recognize collagen, heparin-related polysaccharides, fibrinogen, and/or fibronectin. S. aureus expresses a subset of MSCRAMMs, which all contain the serine-aspartate dipeptide repeat (SDR) domain, including clumping factor A (ClfA), clumping factor B (ClfB), SdrC, SdrD, and SdrE (Becherelli et al. “Protective Activity of the CnaBE3 Domain Conserved Among Staphylococcus aureus Sdr Proteins,” PLoS One 8(9): e74718 (2013)). S. epidermidis also expresses three members of this family, SdrF, SdrG, and SdrH (McCrea et al., “The Serine-Aspartate Repeat (Sdr) Protein Family in Staphylococcus Epidermidis,” Microbiology 146:1535-1546 (2000)). All of these proteins share a similar structure comprising an N-terminal ligand-binding A domain followed by the SDR domain, which can contain between 25-275 serine-aspartate dipeptide repeats. The C-terminal portion of these proteins contains the LPXTG-motif, which facilitates cell wall anchoring by the transpeptidase sortase A. The serine-aspartate dipeptide regions in SDR-containing proteins are modified by the sequential addition of glycans by two glycosyltransferases. First, SdgB appends N-acetylglucosamine (GlcNAc) on serine residues within the serine-aspartate dipeptide regions, followed by SdgA modification of the glycoprotein, resulting in disaccharide moieties. This glycosylation can protect SDR-containing staphylococcal proteins from Cathepsin G-mediated degradation (Hazenbos et al., “Novel Staphylococcal Glycosyltransferases SdgA and SdgB Mediate Immunogenicity and Protection of Virulence-Associated Cell Wall Proteins,” PLoS Pathog 9(10):e1003653 (2013)).
Additionally, Protein A, also located on the surface of S. aureus, contributes to staphylococcal escape from protective immune responses by capturing the Fc domain of host IgG, as well as the Fab domain of the VH3 clan of IgG and IgM (Sjodahl et al., “Repetitive Sequences in Protein A from Staphylococcus aureus. Arrangement of Five Regions Within the Protein, Four Being Highly Homologous and Fc-Binding,” Eur. J. Biochem. 73:343-351 (1997); and Cary et al., “The Murine Clan V(H) III Related 7183, J606 and 5107 and DNA4 Families Commonly Encode for Binding to a Bacterial B cell Superantigen,”Mol. Immunol. 36:769-776 (1999)).
Additionally, S. aureus expresses a second immunoglobulin binding protein referred to as the second binding protein for immunoglobulins (Sbi) (Zhang et al., “A Second IgG-Binding Protein in Staphylococcus aureus,” Microbiology 144:985-991 (1998) and Atkins et al., “S. aureus IgG-binding Proteins SpA and Sbi: Host Specificity and mechanisms of Immune Complex Formation,” Mol. Immunol. 45:1600-1611 (2008)) that is either secreted or associated with the cell envelope (Smith et al., “The Sbi Protein is a Multifunctional Immune Evasion Factor of Staphylococcus aureus” Infection & Immunity 79:3801-3809 (2011) and Smith et al., “The Immune Evasion Protein Sbi of Staphylococcus aureus Occurs both Extracellularly and Anchored to the Cell Envelope by Binding to Lipotechoic Acid” Mol. Microbiol. 83:789-804 (2012)) and shares a pair of conserved helices with Protein A involved in binding to the Fc region of IgG proteins (Atkins et al., “S. aureus IgG-binding Proteins SpA and Sbi: Host Specificity and mechanisms of Immune Complex Formation,” Mol. Immunol. 45:1600-1611 (2008)).
Furthermore, S. aureus secretes a number of proteases that have been implicated in immune evasion. Rooijakkers et al. demonstrated that S. aureus secretion of staphylokinase, a plasminogen activator protein, led to the activation of plasmin that cleaved both surface-bound IgG and complement C3b, ultimately reducing immune-mediated S. aureus destruction (Rooijakkers et al., “Anti-Opsonic Properties of Staphylokinase,” Microbes and Infection 7:476-484 (2005)). S. aureus also secretes the serine protease glutamyl endopeptidase V8 (GluV8) that can directly cleave human IgG1 in the lower hinge region between E233 and L234 (EU numbering (Edelman et al., “The Covalent Structure of an Entire GammaG Immunoglobulin Molecule,” PNAS 63:78-85 (1969), Brerski et al., “Human Anti-IgG1 Hinge Autoantibodies Reconstitute the Effector Functions of Proteolytically Inactivated IgGs,” J. Immunol. 181:3183-3192 (2008)). It was also recently demonstrated that human anti-S. aureus IgGs are rapidly cleaved when bound to the surface of S. aureus (Fernandez Falcon et al., “Protease Inhibitors Decrease IgG Shedding From Staphylococcus aureus, Increasing Complement Activation and Phagocytosis Efficiency,” J. Med. Microbiol. 60:1415-1422 (2011)).
Taken together, these studies indicate that S. aureus utilizes a number of mechanisms that could adversely affect standard IgG1-based monoclonal antibody (mAb) therapeutics, either by directly cleaving the mAb, sequestering the mAb by Protein A binding, or by killing off the very effector cells required for therapeutic efficacy. It is therefore not surprising that presently there are no mAb-based therapies targeting S. aureus that have achieved final approval for use in humans. Thus, there remains a need for methods and compositions that can treat staphylococcal infection, which (i) evade protein A and Sbi binding, (ii) escape staph-induced proteolysis, (iii) can neutralize leukotoxins and (iv) are capable of opsonizing and delivering S. aureus to phagocytes. The present application meets these and other needs.